Life Cycle Environmental Impacts of Wastewater-Derived Phosphorus Products: An Agricultural End-User Perspective

Recovering phosphorus from wastewater in more concentrated forms has potential to sustainably recirculate phosphorus from cities to agriculture. The environmental sustainability of wastewater-based phosphorus recovery processes or wastewater-derived phosphorus products can be evaluated using life cycle assessment (LCA). Many LCA studies used a process perspective to account for the impacts of integrating phosphorus recovery processes at wastewater treatment plants, while some used a product perspective to assess the impacts of producing wastewater-derived phosphorus products. We demonstrated the application of an end-user perspective by assessing life cycle environmental impacts of substituting half of the conventional phosphorus rock-based fertilizers used in three crop production systems with wastewater-derived phosphorus products from six recovery pathways (RPs). The consequential LCA results show that the substitution reduces global warming potential, eutrophication potential, ecotoxicity potential, and acidification potential of the assessed crop production systems in most RPs and scenarios. The end-user perspective introduced in this study can (i) complement with the process perspective and the product perspective to give a more holistic picture of environmental impacts along the “circular economy value chains” of wastewater-based resource recovery, (ii) enable systemwide assessment of wide uptake of wastewater-derived products, and (iii) draw attention to understanding the long-term environmental impacts of using wastewater-derived products.


Influent composition
The wastewater has a typical composition of raw municipal wastewater with minor contributions from industries. Three influent pollutant concentration levels (i.e., "low", "low-medium" and "medium") were defined. The flowrates are similar and only the concentrations of the components are varying. The "low-medium" level corresponds to the BSM2-PSFe 1 default influent composition. The "low" and "medium" levels were derived from the "low-medium" level using a factor of 0.6 and 1.17, respectively, for each influent state variable (except for fermentation products, S A ) to ensure agreement with typical concentration values and ratios found in Volcke et al. (2020), 2 after which the parameter values in Table  S1 were calculated from the influent state variables. The default influent flow rate of 20,935m 3 /d of BSM2-PSFe was used.
Table S1 Values of wastewater parameters according to "low", "low-medium" and "medium" levels.
Parameter  3 with average sludge production (dry weight) All sludge produced in RP3-RP6 is mono-incinerated (highlighted in red).  3 with average sludge production (dry weight) All sludge produced in RP3-RP6 is mono-incinerated (highlighted in red).  3 with average sludge production (dry weight) All sludge produced in RP3-RP6 is mono-incinerated (highlighted in red).  Figure S1 One set of inventories for 1 kg maize production (RP1 to RP3, "low-medium" pollutant level scenario) Figure S2 One set of inventories for 1 kg maize production (RP4 to RP6, "low-medium" pollutant level scenario) Low Land application High RPn-S10 Low-medium Incineration Low RPn-S11 Low-medium Incineration Medium RPn-S12 Low-medium Incineration High RPn-S13 Low-medium Landfill Low RPn-S14 Low-medium Landfill Medium RPn-S15 Low-medium Landfill High RPn-S16 Low-medium Land application Low RPn-S17 Low-medium Land application Medium RPn-S18 Low-medium Land application High RPn-S19 Medium Land application High

S4. Monte Carlo simulation
Monte Carlo simulation was performed on the maize production system by propagating parameter uncertainty (Table S17) for all recovery pathways and their scenarios. The uncertainties within crop production system are excluded because the comparison is between the baseline pathway and the recovery pathways, of which they share the same crop production system. Because of the simulation size, infrastructure processes have also been excluded. Each simulation consists of 1000 runs. The Monte Carlo simulation results are presented as the probability of each resource recovery pathway having lower impact potential than the baseline pathway ( Figure S2). For instance, in the case of global warming potential, RP1 has nearly 100% certainty that the recovery pathway (struvite) across all studied scenarios has lower global warming potential than the baseline pathway. In most cases, the certainty depends a lot on the scenarios (influent pollutant concentration, sludge disposal method, and carbon intensity of grid electricity), i.e., the points are spreading out in clusters.

S5. Sensitivity analysis
In this sensitivity analysis, we assessed how the changes in each of the following inputs would influence the "default" results for maize production relative to the baseline. The default scenario is low-medium influent pollutant concentration, sludge landfill, medium carbon intensity of electricity, 400 km product transportation distance, and RP1 bioavailability of 1, RP2 bioavailability of 0.7, RP3-RP4 bioavailability of 0.9 and RP5-RP6 bioavailability of 1. Bioavailability of recovered products has strong influence across all RPs, while sludge disposal method influences more on the ash-based recovery pathways (RP3-RP6).